The 53-year-old opened up about the new film on the Today show on Friday.
"[I] fell in love with their relationship, Carrie and Debbie and the mother-daughter relationship and actually the whole family dynamic. We want people to... basically to cherish their relationships with their families [because of the film]," he told host Katie Couric.
The documentary, which will air on HBO on Saturday, Jan, 7 at 8 p.m. was initially supposed to focus on the bond the duo had throughout their lives.
"[Carrie] felt like she had to share her mother with the public. She didn't have that intimate time with her mother [as a child]; she was always sharing her," Fisher noted. "They always sang together; they would just start breaking out in song. That was real."
But the film eventually morphed into a sort-of love story about a daughter caring for her mother.
"A lot of the film became about Carrie caring about and caring for her mother… Debbie started filming, and she was fine at the beginning, and then it started to happen, her health."
At one point in the film, Carrie can even be heard saying "I'm concerned because my mother is not feeling well."
Fisher went on to add that he and his team, including co-director Alexis Bloom, were exceptionally moved by how the pair treated one another. He also spoke of the devastation he feels over their passing.
"We were, needless to say, shocked at what happened... We were quite shocked when Carrie went first, obviously."
The 60-year-old author and film star passed away on Dec. 27 from an apparent heart attack. The following day, Debbie died from an alleged stroke.
Since then each has been celebrated at a private, joint memorial service (mourners have included Meryl Streep, Meg Ryan and Gwyneth Paltrow), with many outlets reporting Carrie has been cremated so she can be buried alongside her mother.